1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method, device and computer program product for reselecting a cell in a wireless communication system.
2. Discussion of the Background Art
A basic purpose of selecting a cell is to register a user equipment (UE) in a network and receive a service from a base station. If the intensity and the quality of a signal between a UE and a base station become inferior due to the mobility of the UE, the UE reselects another cell in order to maintain data transmission quality. In the present specification, the characteristic of a physical signal associated with a signal-to-noise interference ratio and the intensity of a signal is called a signal characteristic.
Conventionally, methods for reselecting a cell differ according to parameters associated with a radio access technology (RAT) and a frequency characteristic of the cell participating in the reselection of the cell. The conventional methods include:                Intra-frequency cell reselection: A UE reselects a cell having the same center frequency and RAT as a serving cell.        Inter-frequency cell reselection: A UE reselects a cell having the same RAT as a serving cell and a center frequency different from the serving cell.        Inter-RAT cell reselection: A UE reselects a cell using a RAT different from a RAT which is being used by a serving cell.        
FIG. 1 shows the conventional operation of a UE in an idle mode when the UE is powered on. Referring to FIG. 1, when the UE is powered on, the UE automatically or manually selects a public land mobile network (PLMN) from which a service is desired to be received, and a RAT for communication (S110). The PLMN and RAT information may be selected by the user of the UE or by the PLMN. Alternatively, the PLMN and the RAT information stored in a universal subscriber identity module (USIM) may be used.
Thereafter, the UE performs an initial cell selection process of selecting a cell having a highest value among cells in which the intensity or the quality of a signal measured from a base station is larger than a reference value (S120). The reference value indicates a value which is defined in a system in order to ensure the quality of a physical signal in transmission and reception of data. Accordingly, the reference value may be changed according to the applied RATs. Thereafter, the UE receives system information (SI) that is periodically transmitted by the base station. The SI includes basic information necessary for accessing a network. In addition, the SI may include a neighbor cell list (NCL) associated with cells neighboring with a serving cell. Accordingly, the conventional UE should receive the SI before accessing the base station and should have the newest SI. When the UE is first powered on, the UE selects a cell in order to receive the SI in the idle mode.
The UE registers its information (e.g., international mobile subscriber identity (IMSI)) in a network in order to receive a service (e.g., paging) from the network (S150). The UE does not register its information whenever the cell is selected. Instead, the UE registers its information in the network only if needed. For example, the UE registers its information in the network if network information (e.g., tracking area identity (TAI)) received from the SI is different from the network information about which the UE knows (S140 and S170).
If the intensity or the quality of the signal measured from the base station of the serving cell is lower than the value measured from the base station of a neighbor cell, the UE selects one of other cells providing better signal characteristics than the cell of the base station which the UE accesses. In addition, if priorities are defined in the frequencies or RATs, the cell may be selected in consideration of the priorities (S160). In order to distinguish this process from the initial cell selection process of the step S120, this process (S160) is called a cell reselection process. In a Long Term Evolution (LTE) system, as a signal measurement value, reference symbol received power (RSRP), reference symbol received quality (RSRQ) and received signal strength indicator (RSSI) is being discussed. Long Term Evolution (LTE) is a 4G wireless broadband technology developed by the Third Generation Partnership Project (3GPP), an industry trade group.
In FIG. 1, the basic operation for reselecting the cell according to the signal characteristic by the UE was described. In the LTE, the following parameters are also considered during cell selection.                UE capability        Subscriber information        Camp load balancing        Traffic load balancing        
The UE capability parameter includes an indication that the UE selects the cell according to a selectable frequency band. The UE makes the selection according to the selectable frequency band because the frequency band which is available by the UE may be restricted. The subscriber information parameter includes an identification by the UE of a certain cell to be selected or unselected according to the subscriber information or the provider policy. The camp load balancing parameter includes an indication that a cell which is being used by a small number of UEs is selected in order to reduce a data load generated when UEs in the idle mode are activated in another cell. Similarly, the traffic load balancing parameter includes an indication that a cell is changed in order to reduce a data load generated in activated UEs.
In particular, the LTE system has a possibility that the frequency band expands on the basis of the existing UTRAN for the purpose of installation/maintenance/repair. UEs in the same cell share a radio resource for communication, and thus load balancing between cells needs to be achieved in order to efficiently use the radio resource. Accordingly, the camp/traffic load balancing is defined as a necessary requirement of the LTE system.
In the LTE system, in order to efficiently realize the cell selection process, the priority may be defined per selectable frequency or RAT. Accordingly, the UE preferentially selects a certain frequency or RAT at the time of cell selection or cell reselection. If there is a plurality of frequencies/RATs, the frequencies/RATs may be different from one another, some of the frequencies/RATs may be equal to one another, or all the frequencies/RATs are equal to one another. In the LTE system, only a frequency or RAT having a priority is defined as a cell reselection target.
Hereinafter, an operation for performing conventional cell reselection by a UE according to the priority of the frequency or the RAT will be briefly described. If a signal characteristic value of a serving cell does not satisfy a threshold value (e.g., Snonintrasearch), the conventional UE measures the signal of another cell having a frequency/RAT other than the frequency/RAT of the serving cell. At this time, if a cell using the same RAT as the serving cell and using a frequency having a priority different from that of the serving cell is reselected, the UE has only one restriction time duration (e.g., TreselectionEUTRAN) as a cell reselection criterion regardless of the priority of a candidate cell. That is, a restriction time duration for reselecting a cell of a priority higher than that of the serving cell is equal to a restriction time duration for reselecting a cell of a priority lower than the serving cell. Accordingly, when the conventional UE determines that a time period for which the signal characteristic value of the candidate cell is equal to or greater than the threshold value satisfies the restriction time duration, a probability that the evaluation fails due to radio fluctuation becomes equal in all candidates cells regardless of the priority.
FIG. 2 shows a conventional process of reselecting a cell by a UE, the reselected cell having a different priority. Referring to FIG. 2, when the signal characteristic value of the serving cell is equal to or less than a certain threshold value (e.g., Thresh_serving), the UE begins to perform a search and measurement for reselecting another cell. At this time, the priority of a cell using a first frequency (frequency 1) is lower than a priority of the serving cell and the signal characteristic value thereof is equal to or greater than a certain threshold value Thresh_L. In addition, the priority of a cell using a second frequency (frequency 2) is higher than that of the serving cell and the signal characteristic value thereof is equal to or greater than the certain threshold value Thresh_L. The UE checks whether the cells satisfy the cell reselection criterion during TreselectionEUTRAN.
In FIG. 2, it is assumed that the value of TreselectionEUTRAN is 10 seconds. Accordingly, in order to enable the UE to select the cell using a frequency higher than that of the serving cell, the cell using frequency 2 should have a signal characteristic value equal to or greater than the threshold value (e.g., Thresh_H) for 10 seconds. However, the cell using frequency 2 does not have the signal characteristic value equal to or greater than the threshold value for 10 seconds due to radio fluctuation. When this occurs, a timer for evaluating the signal characteristic value of the cell using the frequency 2 is stopped. In contrast, because the cell using frequency 1 and having the priority lower than that of the serving cell satisfies the signal characteristic value for 10 seconds, the UE reselects the cell using frequency 1, with cell the having the priority lower than that of the serving cell.
If the UE receives a service using the frequency or the RAT having a low priority, the UE periodically or continuously performs a measurement in order to transit to a frequency or RAT having a higher priority. For example, the base station may allocate a highest priority to a certain frequency (e.g., frequency 2) and allocate a low priority to another certain frequency (e.g., frequency 1), for the purpose of load balancing. In this case, the UE receives a service using the frequency (e.g., the frequency 2) having the highest priority. However, if the signal characteristic value of the highest frequency decreases (e.g., due to the movement of the UE), the UE may reselect the cell using the frequency (e.g., frequency 1) having the low priority. In this case, the UE that receives the service from the cell having the low priority continuously perform the measurement in order to select the cell using the frequency (e.g., the frequency 2) having the high priority. That is, the UE should periodically perform the measurement in order to select the cell having the high priority, although the signal characteristic value of the cell which transmits the service is equal to or greater than a certain signal characteristic value. Accordingly, the UE which selects the cell having the low priority may seriously waste power in order to reselect the cell having the high priority.
If priority is used for the purpose of load balancing, the load becomes lower as the priority of the frequency/RAT becomes higher. Accordingly, when the UE selects the cell using the frequency/RAT having the low priority, the bandwidth received from the cell is decreased. That is, when the cell using the frequency having the low priority is selected, the load is relatively high and thus the UE may not sufficiently receive a desired amount of radio resource. Accordingly, it is difficult to ensure sufficient quality of service (QoS) required by the UE. In addition, the bandwidth allocated to the cell cannot be efficiently used.
As described above, if the UE receives the service from the cell using the frequency or RAT having the low priority, it is difficult to provide the sufficient QoS or bandwidth to the user. In addition, since the cell having the high priority is periodically measured, unnecessary power (e.g., battery life) may be expended.